1. Field of the Invention
The present invention relates to the conversion of the size of molecules in a process stream, using an electrical method, and can be used either to combine small molecules into larger molecules or to separate larger molecules into smaller molecules.
2. Discussion of the Related Art
Electrical conversion of molecules of one molecular weight into molecules of another molecular weight has been described in the patent application published as PCT/US88/03228 in WO 89/02868 (Sackinger), corresponding to U.S. patent application Ser. No. 07/485,856, filed Feb. 22, 1990, which is a continuation application of Ser. No. 07/102,361, filed Sep. 28, 1987. In that application, an array of tubular elements with a semiconducting layer beneath their interior surfaces is used as a converter. Electron and ion impact upon the interior surfaces of the tubes is used to fragment neutral molecules absorbed there. Energy for electron and ion impact is provided by an electric field, produced by the voltage applied from one side of the array to the opposite side of the array. Ion impact, fragmentation and ion emission take place on the interior surfaces of the tubes and also ion recombination into new gas species takes place. It can be said that the device operates using electron and ion impact, an electric field, and a very large interior surface area. This can have applications in the petrochemical process industries.
In the petroleum process industries, for many decades a thermal separation process has been applied to crude oil, which is a mixture of hydrocarbons, to separate out various fractions which are subsequently used for specific purposes. For example, diesel fuel, gasoline, naptha, lubricating oils, and asphalts are separated by vaporization and subsequent condensation at appropriate different temperatures and heights in a tower. The resulting yields of refined products are dependent upon the characteristics of the crude oil supplied, which will be different for each petroleum reservoir. The market demand for each category of refined products is variable in time, and therefore the adjustment of the petroleum refining process to changes in input composition and output demand can be difficult and challenging.
One remedy which has been recognized for many years is to convert larger molecules into smaller ones, a process called cracking, which has been normally accomplished by catalytic action on special surfaces at high temperatures. In this way, for example, an extra quantity of heavy oils can be converted into gasoline, which may be more marketable. Moreover, light naptha, which is a mixture of pentanes and hexanes, may be catalytically cracked to produce ethylene, C.sub.2 H.sub.4, a building block molecule for many petrochemicals. Limitations of this approach include the cost of catalysts, the range of input molecules for which they are effective, and the thermal energy required for the process, which is difficult to recover. In addition, the world supply of liquid hydrocarbons is also being consumed, leading to higher prices for liquid feedstocks. Obvious alternatives include the utilization of other natural sources of hydrocarbons which are in more abundant supply: natural gas, and heavy oil. It is clear that an important advantage would be derived from methods to convert natural gas (methane in particular) into higher hydrocarbons such as ethylene, propylene, and larger molecules. Most processes rely on the high-temperature combination of methane and steam to produce hydrogen and carbon monoxide. For example, a Davy-McKee reformer furnace in New Zealand operates at 880.degree. C. and 30 bar pressure with a nickel catalyst. After cooling to 35.degree. C. at 17 barg pressure, and removal of excess steam, the reformed gas is compressed to 100 barg and passed over a ZnO/CuO/Al.sub.2 O.sub.3 catalyst at 210.degree.-240.degree. C. The output product is crude methanol containing 17% water. An alumina catalyst is then used (at 310.degree.-320.degree. C. and 26 barg pressure) to create dimethyl ether. Clearly, high operating temperatures, high pressures, and costly equipment are necessary. An approach which involves a single-stage direct conversion, and in which the energy which is to be supplied to a reaction is specifically used to accomplish the molecular conversion, with a minimum waste as heat, would represent an improvement.
The electrical method briefly described above offers significant improvements in that low pressures (such as 0.001 to 0.1 bar) and room temperatures are involved. The use of oxygen is not essential, and water need not be part of the reaction unless specified output products containing oxygen are desired.
A limitation on the electrical device, however, is that a lower throughput of molecules is inherent at lower pressures, so that an equipment designer will naturally attempt to choose the operation pressure of the electrical device to be as high as possible, consistent with a specific conversion reaction. Each impact of ion and electron on the tubular reaction surfaces will result in a portion of the impact energy being converted into thermal energy in the material beneath the surface. A large value of pressure implies more ion impacts and more conversion reactions per second, and more heat (random thermal energy) generated in the material. The semiconducting material on the inside surfaces of the tubular elements may, in certain cases, have a negative temperature coefficient of resistance, so that high pressure operation would lead to heating, lower resistance, and higher leakage currents through the semiconductor layers. This may lead to self-heating and thermal runaway of the semiconductor layer, followed by destruction of electrical continuity through the layer. Proper choice of materials can prevent this effect, but it is the object of this invention to provide an alternative array of electrodes so that the semiconducting layer is not needed. Thus, higher pressure and higher throughput operation is facilitated.
It is also possible that in the aforementioned electrical device, operating at high pressures, that the high density of ions in the high-pressure ends of the tubular elements could give rise to an electric field distribution which would depend upon space charge density, which in turn could lead to plasma instabilities and the onset of acr discharge phenomena. In an ideal device, the intensity of the electric field strength would be controlled by external means, and would be time-variable and spatially-variable according to an adaptive and progressively more optimal pattern of applied external voltages. With the tubular elements and their semiconductor layer, this is difficult to accomplish inasmuch as only one voltage is imposed across the ends of the tubes. With this invention hereinafter described, this limitation is removed.
Although the foregoing discussion of molecular conversion is oriented towards hydrocarbon gaseous conversion, it is obvious that compound gases containing other elements, such as oxygen, chlorine, flourine, bromine, nitrogen, sulfur, hydrogen, silicon, and other elements could be changed in composition in order to accomplish certain benefits, such as the diminution of toxicity prior to release into the natural environment, or the alteration of composition so as to render harmful gases into harmless liquids or gases prior to release into the environment. Accordingly, it is an object of this invention to provide an electrical device which will accomplish the function of combining molecules of low molecular weight into molecules of higher molecular weight. A further object of the invention is to provide an electrical device for separating molecules of higher molecular weight into other molecules of lower molecular weight. Moreover, an additional object of the invention is to provide an electrical device for combining two or more different molecular species into new molecular species.